Rep. Joaquin Castro knows a little about real estate, in part because his twin, Julian, is secretary of Housing and Urban Development. So after winning a safely Democratic seat three years ago, he decided buying a condo on the Hill was a smart investment.

He soon started noticing, when walking home at night, how many windows in the darkened House office buildings emanated a distinctive bluish light. Castro guesses the number of his colleagues inside, watching TV while falling asleep, has only grown and easily tops 50 these days.

“I’ve been joking with my brother,” the San Antonio congressman says with a broad grin, “that he should have HUD designate this place a public housing project.” Humor aside, there’s a strong argument to be made for the conclusion. The dozens of members who sleep in their offices are, in effect, spending their weekdays in 100 percent federally subsidized apartments.

They are not charged any rent. They receive no utility bills. They don’t pay for the daily cleaning services. Microwaves and refrigerators have been installed in most suites. There’s a half bathroom connected to each personal office, and the showers at the members’ gym open at 5:30 a.m.

In short, lawmakers who choose their offices as their crash pads are getting a valuable government freebie — worth 10 percent or more of their $174,000 annual salary .

Zillow, the real estate search engine, on Wednesday listed 50 studio or one-bedroom apartments for lease on Capitol Hill, a solid majority for between $18,000 and $23,000 a year. (The cheapest, 447 square feet on 15th Street Southeast, was $1,100 a month.) Airbnb was offering more than 300 spots in the neighborhood for shorter-term rent, at an average of $152 a night. Even with that arrangement, House members would need to budget $17,000 for lodging on the 110 or so weeknights their schedule says they can expect to be in D.C. this year.

The fact that members are free to freeload off Congress is back in the news this week , as the House voted to maintain the congressional pay freeze for a seventh year. The few lawmakers willing to advocate for a raise inevitably lamented the expense of maintaining places to live in both their districts and high-cost Washington. Those complaints, in turn, revived chatter about the growing numbers who have figured out how to avoid the problem.

No Capitol administrative office claims to keep track of which members bunk next to their desks, but news stories and anecdotal reports suggest the number is at least 45 and may be 70 or higher. No senators appear to be doing so, and the House contingent seems to be exclusively male — and lopsidedly Republican.

That partisan skew makes complete sense on one level, but is strikingly counterintuitive on another.

Boasting about sleeping in the office is an easy way for the younger generation of fiscally conservative tea party Republicans to signal both their prudence and their disinterest in becoming “Washington insiders.”

At the same time, a central aspect of GOP orthodoxy is that too many Americans have become reliant on federal largess when they could be out working — “takers, not makers,” in one often-used construction. (Mitt Romney infamously pegged at 47 percent the share of the population “who believe the government has a responsibility to care for them, who believe that they are entitled to health care, to food, to housing, to you-name-it.”) It hardly seems a stretch to put into that group the Republicans who belong to the congressional Couch Caucus (as The New York Times first dubbed the office dwellers).

But the practice was pioneered in the 1980s by one of the most prominent House GOP free-marketers of the past several decades, House Majority Leader Dick Armey of Texas, who bedded down in Cannon 514 until Speaker Jim Wright forbade it as demeaning to the institution of the House.

Neither the current GOP leadership nor the House Ethics Committee has raised any similar objections.

Four years ago, the government watchdog group Citizens for Responsibility and Ethics in Washington asked the Office of Congressional Ethics to investigate whether members who sleep in their offices violate House rules and federal tax law.

The group noted House prohibitions against using “official resources” for anything other than official business, and against spending a member’s official allowance on personal expenses. And, since IRS rules say lodging is generally a taxable fringe benefit, CREW argued, members sleeping in Cannon, Longworth or Rayburn should pay taxes for imputed income based on the fair market value of a comparably sized Capitol Hill apartment.

Nothing ever came of the complaint. The independent ethics office either dismissed the allegations or forwarded a recommendation to the House Ethics Committee and that panel chose to drop the matter. (In either case, there wouldn’t have been any public announcement.) And House Ethics, which has “pink sheets” offering members guidance through a range of behavioral gray areas, has no document explaining the do’s and don’ts of combining a congressional office and a home away from home — suggesting the committee views the practice as unambiguously above-board.

For many staffers, the situation isn’t so great. Sometimes arriving for work early and encountering the boss before he’s brushed his teeth is just awkward. Other times, it’s arguably a hostile work environment when you’re forced to confront the pajamas and mussed hair of some of the most important policymakers in the land.

But for the members themselves, it seems, there’s only one hassle. They must tidy their rooms every morning — stashing their bed rolls, air mattresses, futons or roll-away cots before constituents or lobbyists arrive for the first meeting of the day.

If they actually use the office sofa, a more intense cleanup may be in order — not so much to cover up any ethical questions or ideological inconsistencies, but to put their guests at ease.